This work will characterize further the factors necessary for quantitative understanding of water-proton nuclear spin relaxation in heterogeneous systems such as tissues. A theoretical description of the magnetic relaxation coupling between a mobile water spin and rotationally immobilized protein spins that permits predictions about the magnetic field and viscosity dependence of intermolecular magnetization transfer will be developed. The dependence of the effective magnetization transfer rates on the ratio of the solid to the mobile spin population where there is currently an inconsistency with simple models will be examined. The efficiency of exploiting -magnetization transfer in radio frequency fields that may be easily controlled as a potential means for controlling the magnetization transfer rates in analogy to now standard heteronuclear solids experiments will be examined. The extent to which the rotational mobility and other aspects of a heterogeneous sample constructed from purified proteins may mimic or fail to mimic the transverse relaxation behavior of tissue will be studied. The magnetic field dependence of the water proton spin-lattice relaxation rates in tissues is determined by the field dependence of protein proton spins. Therefore, the origins of the magnetic field dependence of protein proton spin relaxation will be examined using both magnetic field cycling measurements of spin-lattice relaxation rates and the rf field dependence of Tlp. Studies are proposed aimed at determining the amplitude and frequencies of structural fluctuations in proteins using deuterium solid state NMR methods to detect different motional domains in proteins, and chemical shift tensor and deuteron quadrupole powder pattern averaging. Investigations of substrate analog motions in protein active sites will be carried out using deuterium, phosphorous, carbon, and nitrogen NMR, and to investigate the interaction between water content and the resolution possible in magic angle spinning NMR experiments that may be used to deduce interatomic distances in proteins.